While much attention has been given to the stabilization of voltages, less regard has hitherto been paid to the stabilization of currents In a number of applications it is, however, primarily a stable current that is required, for example, when the supply is from current sources within a bipolar integrated circuit and with certain types of digital-to-analog and analog-to-digital converters. It is, in fact, possible to derive stable currents from a reference voltage source, but this always involves additional expenditure and a loss of accuracy. From a technical standpoint, the interest in means and methods for stabilizing currents is, therefore, considerable.
Band gap stabilization which goes back to R. J. Widlar (IEEE Journal of Solid-State Circuits, Volume SC-6, No. 1, 1971) relates to voltage stabilization. The parameters it attains are more or less as good as those of the Zener diode stabilization which had been predominantly used until then, smaller supply voltages are adequate, and it can be advantageously employed within a bipolar semiconductor circuit. The core of the circuit consists of two transistors whose current, densities are kept at a certain ratio by a skillful circuitry adjustment. The resulting difference in the voltage of the base-emitter diodes is proportional to the absolute temperature and is fed to a resistor arranged at the emitter of the transistor with the smaller current density, with the result that the current intake of the two transistors is proportional to the absolute temperature. U.S. Pat. No. 4,059,793 discloses that this resistor may also be advantageously arranged between base and collector of the transistor with the higher current density. J. E. Hanna indicates in U.S. Pat. No. 4,091,321 that a current with a freely settable temperature coefficient can be generated within this basic arrangement. This is achieved by a resistor being connected in parallel with a transistor of the band gap circuit which carries a current proportional to the absolute temperature. The current intake of this resistor is proportional to the base-emitter voltage which has a negative temperature coefficient. The sum of the two currents, therefore, consists of a temperature-dependently rising and a dropping current. Independency of temperature can be achieved by weighting. Since the aforementioned Patent deals with the generation of temperature-stable voltages no indication is given as to how to exploit this effect to produce temperature-stable current sources.